Carbon sink reduction by fruit removal triggers respiration but not nitrous oxide emissions from the root zone of cucumber

Root exudation of organic carbon (C) is generally believed to be the cause of positive effects of root activity on nitrous oxide (N 2 O) emissions. We tested the effects of root exudation in an actual soil–plant system on N 2 O emissions while excluding most other potential factors. The C source/sink ratio in cucumber was changed by removing fruits to increase root exudation. Root‐zone emissions of carbon dioxide (CO 2 ) and N 2 O were monitored in complete stands of adult plants in a greenhouse. Whereas CO 2 emissions rapidly increased as a result of fruit removal the N 2 O emissions were completely unaffected. After cutting the shoots CO 2 emissions decreased within 2 weeks in both the fruit removal treatment and the control to a value significantly lower than that before the start of the treatments. However, N 2 O emissions immediately exhibited a short peak, which was significantly higher in the fruit removal treatment compared to the control. Thereafter N 2 O emissions in both treatments remained on the same level but considerably higher than before shoot cutting. We concluded that in a well‐aerated root zone, a root exudation pulse does not necessarily increase N 2 O emissions, because C substrates are quickly respired by microorganisms before they can support heterotrophic denitrification. The results further indicate the significance of dying/dead roots for the creation of denitrificaton hot‐spots, which likely result from providing C substrates as well as poorly aerated habitats.